Generally, in the case of a digital video camera, the signal outputted from a CCD (Charge Coupled Device) of a picture pickup is an analog signal. This analog signal is converted to a digital signal for signal processing. The signal processing is performed after the digital signal is fixed on a predetermined level by a clamp means.
A digital clamp circuit is connected to receive an output of an A/D converter (Analog to Digital Converter) which converts the analog CCD output to a digital signal in a digital camera. After the black level detected from the output signal and a reference black level are compared with each other, the digital clamp circuit compensates and clamps as much as the difference thereof against a signal component of 1 H period.
In other words, in the case of a digital video camera, an analog method is applicable to a CCD of a picture pickup, the output level of which can thereby be changed per 1 H (1 horizontal period).
Accordingly, the output level is compensated based on a value of an Optical Black Period set up per 1 H by the clamp circuit.
However, if the entire level of 1 H period is changed, the value of an optical black period also changes, thereby requiring compensation thereto. Specifically, a dark current is included in the output circuit of the camera tube, and this dark current varies in response to ambient temperatures. As a result of the variation of the dark current, the black level changes, and the balance of the black region in the image signal also changes, thereby decreasing the picture quality. The dark current can be represented in an optical black level during the optical black period which necessitates the optical black level to be fixed on a predetermined level at all times. However, when the overall level of 1 H period changes, the optical black level of the optical black period also changes, thereby requiring compensation.